With the constantly improving quality of color photocopies and printings and in an attempt to providing security documents such as banknotes, value documents or cards, transportation tickets or cards, tax banderols, and product labels that have no reproducible effects and thus protecting them against counterfeiting, falsifying or illegal reproduction, it has been the conventional practice to incorporate various security means in these documents. Typical example of security means include security threads, windows, fibers, planchettes, foils, decals, holograms, watermarks, security inks comprising optically variable pigments, magnetic or magnetizable thin-film interference pigments, interference-coated particles, thermochromic pigments, photochromic pigments, luminescent, infrared-absorbing, ultraviolet-absorbing or magnetic compounds.
Coatings, printings and markings exhibiting a viewing-angle-dependant visual appearance and colorshift properties are used as efficient anti-copy means on banknotes and security documents (cf. “Optical Document Security”, ed. R. L. van Renesse; 2nd edition, 1998, Artech House, London). Optically variable inks are formulated on the base of optically variable pigments (OVPs), preferably flakes of the thin-film optical interference device disclosed for example in U.S. Pat. No. 4,434,010; U.S. Pat. No. 5,084,351; U.S. Pat. No. 5,171,363; EP-0 227 423 A2 and in related documents.
Non-optically variable magnetic pigments or flakes have been used as security features. For example, JP-A 06-297890 discloses non-optically variable magnetic pigments that are specifically oriented in a way to provide an anti-photocopy security features.
As an upgrade from optically variable ink, optically variable magnetic pigments have been specifically designed and used in security applications. Optically variable magnetic pigments which can be used for security purpose have been disclosed in U.S. Pat. No. 4,838,648; in EP-0 686 675 A1, WO 2002/073250 A2 and WO 2003/000801 A2. Corresponding inks and coating compositions have been disclosed in WO 2007/131833 A1.
WO 2005/002866 A1 discloses optically variable printed security elements, comprising a high-resolution image represented by oriented optically variable magnetic flakes in a solid or hardened coating layer. The high-resolution image is produced by using a particular magnetic orienting device. The security document is first imprinted with an ink or coating composition comprising magnetic or magnetizable particles, such as optically variable magnetic pigment flakes. The imprinted document is subsequently exposed, while the coating is still “wet”, to the magnetic field of the magnetic orienting device, comprising a magnetized permanent-magnetic plate engraved with indicia. The magnetic or magnetizable particles of the coating are oriented under the influence of the magnetic field of the orienting device, hereby forming an image of said engraved indicia. The coating is subsequently hardened, so as to freeze the magnetic or magnetizable particles in their positions and orientations.
Whereas several printing processes are available for applying an ink or coating composition comprising optically variable magnetic flakes, WO 2005/00585 A1 discloses that the most convenient method consists of a silkscreen printing process. Indeed, silkscreen printing ensure that the flakes are not destroyed or damaged during the printing process and has the advantage that the inks or varnishes used in such a process exhibit a relatively low viscosity which favors proper and fast orientation of the magnetic flakes.
U.S. Pat. No. 7,449,212 B2 discloses a security element comprising a material that is optically changeable by an electric or magnetic field. The optically changeable material is enclosed in microcapsules containing a swelling agent and incorporated in a binder. The so-obtained security element may be incorporated into a security document and may be switched within the dried or cured binder from one optical state to another optical state. Consequently, it is possible to change the form of appearance of a printed character solely by moving the security document within an electric or magnetic field.
Therefore, a need remains for ink compositions comprising magnetic of magnetizable particles that are easily and fast oriented in ink compositions.